Try this way... Display your form in a browser, click right mouse button -> inspect on the fields inside repeater, Look at field id in browser dev tools and adjust the script accordingly. 

Let me see if I understand your question. You want to have a lookup field and a second field watching the lookup field in a repeater, is that correct? I don't see why this wouldn't work, have you tried it? The Formidable Knowledgebase is quiet on the issue, but a repeater is an embedded form. Since lookup/watch fields work on a parent form, it should work on an embedded form as well. The simplest thing to do is try it and see if it works.